should not be running it on 5w30 !! to thin a oil for it
should be 5w40 or 15w40
if it is the std turbo should be running a t25 not t28 less been upgraded
dont think running lower comp pistons would make much of a difference to the turbo just alouds you to run slight more boost etc

surgest you change the oil check all the oil lines to the turbo (make sure no bits of metal or debree in the line cos it wont last long if there is) and get the car set up properly

and like mentioned by wilky get the turbo spec by turbo specialist and give it the once over and upgrade any parts if nessacessary

(sorry bout the spelling )

why dont you have a chat to stu at motorsport developments see if he can guide you

www.motorsport-developments.co.uk or 1253 508400 blackpool